Hi-
I use my mini 5+ hours per day, with sleep intervals when I'm not on my system. At the end of my last session I do a standard OSX shutdown sequence; leaving my power strip and mini's power brick on to ensure the the EPROM (date/time/parameters) battery stays charged. What is your experience? Do you let your mini run 24/7? Do you just let it sleep, rather than shutting it down?

I sleep mine. My Mac mini doesn't like to shut down, as a matter of fact, it wont. When I try to shut down my mac from doing a OnyX cleanup, or software update, it sits on the blue screen with the wheel spinning. I don't know how to fix this, and hopefully the next firmware update will be the cure. I shut down/restart my mini once a week, hence I do an OnyX cleanup once a week. That is basically my only problem with my mac, and luckily it's not a big one.

I don't think it's an issue. I don't work the machine hard. Drives are replaceable and it is not likely that it will die within the life of the system. My mini is in a clean, smoke free environment.

I also have two good UPS's, the one the mini is plugged into also has power conditioning. This is probably much more important than putting the computer to sleep.

Definately. It's not likely at all your machine will die on you. It's just you stated you wanted to sell your current machine, and get enough money to upgrade and get a new machine with Leopard and iLife '07. Drives are replaceable, and since your mini has treated you well for the time you've had it, I doubt anything bad will happen now.